Cyclical xerographic process utilizing a selenium-tellurium xerographic plate



NOV- 28, i R. HALL ETAL CYCLICAL XEROGRAPHIC PROCESS UTILIZING ASELENIUM-TELLURIUM XEROGRAPHIC PLATE Filed May 2, 196 2 Sheets-Sheet 1JNVENTOR. RICHARD HARRISON HALL MORTIMER LEVY ATTORNEY Nov. 28 1967Filed May 2, 196;

QUANTUM EFFICIENCY (INCIDENT LIGHT,

PERCENT) RELATIVE SPECTRAL RESPONSE PLATE POTENTIAL (VOLTS) CYCLICALXERCGRAPHIC PROCESS UTILIZING A R H. HALL ETAL' 3 5,289

SELENIUM-TELLURIUM XEROGRAPHIG PLATE 2 Sheets-Sheet 2 O I I I 400 500600 v 700 WAVELENGTH (M'ILLIMICRONS) Fly. 3

WAVELENGTH (MILLIMICRO'NS) fl's fl l (8 Hi5 LOG EXPOSURE ]NVENTOR Fig 5RICHARD HARRISON HALL MORTIMER LEVY ATTORNEY United States Patent3,355,289 CYCLICAL XEROGRAPHI PROtIESS UTI- LIZING A SELENliUM-TELLURHUMXERO- GRAPHIC PLATE Richard Harrison Hall, Syracuse, and Mortimer Levy,Scarsdale, N.Y., assignors to Xerox Corporation, Rochester, N.Y., acorporation of New York Filed May 2, 1962, Ser. No. 191,818 2 Claims.(Cl. 96-]..4)

This invention relates in general to the art of xerography, and inparticular to an improved xerographic plate for use in the xerographicprocess. More specifically, the invention relates to an improvedxerographic plate comprising a conductive backing having on at least onesurface thereof a charge carrier layer of specially treated vitreousselenium on which there is a thin photoconductive overlayer consistingof an alloy of selenium-tellurium, and to an improved method ofXerography, therewith.

In the art ofxerography, as disclosed in Carlson U.S. Patent 2,297,691,it is usual to form an electrostatic latent image on a xerographic platewhich comprises a conductive backing member such, for example, ametallic surface having a photoconductive insulating layer thereon. Ithas previously been found that a suitable plate for this purpose is ametallic member having a layer of vitreous selenium. Such a plate ischaracterized by being capable of receiving a satisfactory electrostaticcharge and selec- .tively dissipating such a charge when exposed to animage pattern of activating radiation, and, in general, is highlysensitive to light primarily in the blue, green spectral range.

From this description of the basic process of xerog- .raphy, it isevident that the unique properties of the photoconductive insulatinglayer are critical for the successful operation of the process. Thislayer must have a resistivity of at least about ohms-cm. in the dark inorder to retain an electrostatic charge on the surface for a significantperiod of time and desirably it has a resistivity even greater thanthis. It must also have the property of lowering its resistivity onactivation by radiation. The discovery of the photoconductive insulatingproperty of highly purified vitreous selenium has resulted in thismaterial becoming the standard in commercial xerography. It hasoutstanding light-sensitivity, and is also outstanding in its darkresistivity which is at least about 10 ohms-cm.

Since selenium was first employed as a xerographic plate conductor, muchprogress has been made in the understanding of solid state physictheories and mechanisms applicable to such use. Briefly, it has beenfound that vitreous selenium conducts both electrons and holes and thatthe mobility for holes is approximately ten times that for electrons.Thus, vitreous selenium, while possessing a long range for holes, has avery short range for electrons. Sostrongly does amorphous selenium trapelectrons that Weimer in U.S. Patent 2,687,484 states that the materialhas little or no electron conductivity. Accordingly, it has become thestandard in xerography to use vitreous selenium layers with positivepolarity sensitizing charges thereon.

Advances in the art have disclosed xerographic plates with increasedspectral sensitivity particularly toward the red end of the spectrum aswell as generally increased sensitivity or speed. As disclosed inUllrich, U.S. Patent 2,803,542, improved plate properties are obtainedutilizing a mixture of arsenic and selenium. In Mengali U.S. Patent2,745,327, there is disclosed improved plate properties utilizing aphotoconductor comprising a mixture-of selenium with tellurium, whereasin Paris U.S. Patent 2,803,541, a mixture of selenium and tellurium isutilized in a thin layer overcoating the photoconductor of predominantlyvitreous selenium. Each of these known structures when incorporated intoa xerographic plate enhances the photoconductive properties in what isregarded as a highly desirable area, i.e., increased speed andpanchromaticity.

However, despite these advantages, xerographic plates other thanessentially pure vitreous selenium have not enjoyed commercial success.One reason for their lack of commercial success has been the inabilityheretofore to reproduce these plates reliably with consistency ofcharacteristics within tolerable ranges. In addition, these plates underother than laboratory conditions, i.e., under continuous use andrecycling experienced in the xerographic process, experience relativelyhigh dark decay, relatively high residual potential, and high orders offatigue as compared to photoconductive layers of commercially availablevitreous selenium. By this, it is meant that these alloyed plates of thetypes referred to generally suffer from an increased impairment ofdesirable operating propcities in successive cycles of the xerographicprocess. In the early period of xerography it was common to employmanual processing procedures in connection with a set of individualxerographic plates. Under these conditions each plate was reusedrelatively infrequently. Now, however, Xerography is increasinglyconcerned with automatic processing equipment employing a singlecontinuously rotating cylindrical xerographic plate. In such machineseach portion of the sensitive surface of the plate may be repetitivelyrecycled as many as about 10 times per minute. Thus, the recyclingrequirements for xerographic plates have become very severe.Furthermore, as stated above, these plates of the prior art, includingordinary pure vitreous selenium, have been limited to utilization withinitial sensitizing charges of positive polarity because of their longrange for holes While having a short range for electrons such that theyare unsuitable for sensitizing with charges of negative polarity.

Now in accordance with the present invention there is provided animproved xerographic plate of specially processed selenium with anoverlying selenium-tellurium alloy layer, having increased speed withincreased spectral sensitivity and good reproducibility and adapted forrepetitive cycling. There is also provided an improved method of usingsuch a plate in repetitive cycles whereby the plate is negativelycharged before exposure and positively charged before reuse. Theadvantages to xerography of such a plate are many, offering opportunityfor increased utility by increased spectral. response while permittingmore flexibility and more efiicient and rapid performance of thexerographic process.

It is therefore an object of the invention to provide improvedxerographic plates having increased speed and sensitivity and adaptedfor cyclic reuse.

It is a further object of the invention to provide a xerographic platewith increased spectral sensitivity that can be utilized withsensitizing charges of negative polarity.

It is a further objective of the present invention to provide axerographic plate having a layer of specially processed selenium and athin layer of selenium-tellurium alloy coated thereover.

It is a further objective of the invention to provide an improved methodof operating xerographic plates in repetitive cycles. 1

These and other objects are attained in accordance with the inventionutilizing a xerographic plate comprising a conductive backing memberhaving thereon a charge storage layer of vitreous selenium that has beenprocessed in a manner to be described, and an upper thin photoconductivelayer containing a seleniu'm-tellurium alloy as hereinafter described.The plate when being utilized in the xerographic process in which it issuccessively and repetitively subjected to charging, exposing, anddeveloping, has been found to be particularly superior when employed ina manner to be described. More specifically, when employed in thismanner, a plate is repetitively cycled in a process including negativecharging, exposure to an image pattern of radiation, development,transfer, cleaning, and positive charging with concomitant uniformillumination.

In the drawings:

FIG. 1 isometrically illustrates a xerographic plate in accordance withthe invention;

FIG. 2 schematically illustrates in section an apparatus arrangement inaccordance with the invention; and

FIGS. 3, 4, and 5 are graphs which illustrate characteristic propertiesof a xerographic plate in accordance with the invention compared tocharacteristics of a plate of the known art.

Referring to FIG. 1 there is illustrated a xerographic plate comprisinga conductive backing member 11 on which there is supported a chargestorage layer 12 which in accordance with the invention is speciallyprocessed amorphous selenium, and on top of which is a thinphotoconductive layer 13 of a selenium-tellurium alloy. Because of thethinness of layer 13 and in order to protect it from wear, a thin butdurable and transparent insulating overcoating 14 of a type to bedescribed is preferably although optionally applied over layer 13 solelyfor mechanical protection.

Referring to FIG. 2, the plate 10 is shown cylindrical in form andadapted to be continuously rotated by a motor M-1 in the directionindicated by the arrow. As the drum rotates, an electrostatic charge isapplied to its surface by a corona generating device which may be of atype discolsed in Vyverberg US. Patent 2,836,725 and connected to asource of high potential 21. The charge applied by generator 20 ispositive in polarity and. on the first cycle of operation is termedpregeneration whereas thereafter for subsequent cycles is termedregeneration, as will be understood by the description to follow. Theregion of the plate being charged by generator 20 is concomitantlyexposed by a source of activating radiation such as illumination source22, that suitable may be an incandescent lamp, fluorescent lamp or thelike. A light shield 19 prevents light from reaching the drum past theimmediate vicinity of the lamp.

Next in the direction of rotation, the drum surface passes a coronagenerator 23, similar to generator 20 mentioned above, which applies asensitizing potential of negative polarity on the drum surface usuallyon the order of -300 volts to 1000 volts. After charging, the surface isexposed to a light image of copy 24 being advanced from a supply reel 25to a takeup reel 26 at a rate proportional to the rate of drum rotation.As the copy advances past the optical axis of objective lens 27, it isilluminated by lamps 28 and projected by the lens through a an exposureslit 29 incrementally onto the drum surface. This selectively causesdissipation of the image charging potential to form an electrostaticlatent image in image configuration of the original copy.

The electrostatic latent image is developed by a developing apparatus 40that may be of a suitable type known to those in the xerographic art asfor example disclosed in US. Patent 2,945,434 and which may utilize apowdered resin that is cascaded over the drum surface and iselectroscopically attracted to the electrostatic latent image to renderit visible. The developed image may then be transferred to a secondarysupport surface 41, which may be paper or the like, drawn from a supplyreel 42 onto a takeup reel 43 being driven also from motor M-1. Thesupport surface contacts the drum in the vicinity of corona generatingdevice 44, which is similar to corona generating devices 20 and 23mentioned above, and which applies an electrostatic charge ofappropriate polarity to thelsback of the paper for effectivelytransferring the developed image from the drum to the paper. Thereafterthe support surface passes through a suitable fusing apparatus 45 whichmay be a heat fuser of the type disclosed in Crurnrine US. Patent2,852,651 or a vapor fuser of a type disclosed in Carlson US. Patent2,776,907 wherein the loosely supported developed image on the supportsurface is permanently affixed thereto. The drum continues to pass to acleaning station at which a brush 46 or the like cleans the drum inpreparation for recycling. Subsequently the plate passes by generator 20which applies opposite polarity to that applied by 23, but preferably ofsubstantially similar order of magnitude.

The general nature of the invention having been set forth forillustrative purposes only, the following examples are now presented asillustrations but not limitations of the methods and means for carryingout the invention. in the following examples, the plates were preparedon an aluminum substrate by vacuum evaporation of the photoconductivematerials while the base was held at C., in each case the substratehaving first been processed by heating in a solution of 3% sodiumcarbonate and 3% trisodium phosphate at C. for 45 seconds after which itwas rinsed twice in distilled water and then heat treated and allowed todry at 200 C. for 105 minutes.

Example 1 A 40 micron charge storage layer of specially processedamorphous selenium was vacuum evaporated as a base onto the aluminumsubstrate and a 0.1 micron layer of selenium-tellurium alloy wasevaporated thereover without breaking the vacuum. Processing of theselenium included placing substantially pure selenium in a glass ampulewith an amount of titanium wire equal to approximately 7 /2 percent ofthe weight of selenium and placed at one end of the ampule. The ampulewas evacuated to 20 to 30 microns of mercury pressure, sealed and thenheated in an oven at approximately 525 F. for two hours after theselenium had melted, with constant contact between the molten seleniumand the titanium wire. At the end of the treatment, the ampule wastipped so that the molten selenium drained away from the wire and thencooled. The ampule was broken and the selenium broken up and placed inthe evaporating crucible. A seleniumtellurium alloy comprisingapproximately 25% by weight of tellurium was prepared by heating thematerials together for approximately two hours in a vacuum sealed glasstube 450 C., cooling, and pulverizin-g. After evaporation of theselenium, small increments of the pulverized alloy were dropped into aseparate crucible, heated to 500 C. until the 0.1 micron alloy layer wasobtained. The resulting xerographic plate Was then cooled, removed fromthe vacuum and dipped in a solvent solution of nitrocellulose to providean abrasion resistant protective layer approximately 2 microns inthickness.

Example 2 A glass tube containing 50 grams of selenium and 20 grams ofanalytical reagent iron wire of a type marketed by the MallinkcrodtChemical Company was evacuated, sealed and heated to 350 C. for 24 hoursafter which the contents were vacuum evaporated onto an aluminumsubstrate to form a 40 micron layer of vitreous selenium. A mixture of25 parts tellurium and parts selenium by weight was processedidentically as in Example 1 to form an alloy and was similarly vacuumevaporated onto the vitreous selenium layer. A transparent protectiveovercoating of nitro-cellulose approximately 2 microns thick was thenapplied over the tellurium-selenium layer similarly as in Example 1.

Example 3 A plate was prepared similarly as in Example 2 except that theselenium used for the layer of vitreous selenium was processed in aglass tube with the same weight of selenium but with 50 grams ofanalytical reagent iron wire.

Selenium layers which are specially processed in accordance with theprocedures illustratively illustrated in Examples 1, 2, and 3 arecharacterized as having a low dark discharge rate when negativelycharged and a high range and mobility for negative charge carriers asdemonstrated by substantial light sensitivity when negatively charged,although light sensitivity is not employed per se in the presentinvention. Each plate thus prepared was then subjected to a series oftests to determine various characteristic properties of spectralsensitivity, spectral response and electrical contrast, each to bedescribed. The test for spectral sensitivity consisted of exposingthrough a slit to the previously sensitized plate from an enlarger witha No. 2 photoilood incandescent lamp and interference filters at 400,500, 600, and 680 millimicron wavelength. The plates were sensitizedwith charge of negative polarity. Light intensity was measured with acalibrated Densichron and the plates were tested after numerousrepetitive cycles of charging and exposure and termed 'to be in awell-cycled condition. A commercial selenium plate obtained from XeroxCorporation, of Rochester, New York, was used as a standard comparisontest plate. To obtain the best results with this plate, it wassensitized with positive polarity as recommended by the manufacturer.The spectral sensitivity of plates of the examples were foundsubstantially similar to each other, although the plate of Example 3showed slight superiority having slightly increased spectral sensitivityover the plates of Examples 1 and 2.

Referring to FIG. 3, the spectral sensitivities of the various platesare graphically illustrated in terms of quantum efliciency plotted asthe ordinate against wavelength as the abscissa. By quantum efliciencyis meant that plate property consisting of the measured flow of chargein excess of dark current per photon of light energy incident on theplate and expressed as a percentage. Curve A represents the test plate(i.e., the commercial selenium plate) while curve B represents the plateof Example 1 and curve C represents the average of the plates ofExamples 2 and 3. From these curves it may be seen that platesconstructed in accordance with the invention and represented by curves Band C have increased spectral response, the sensitivity extending to 700millimicrons wave length, as compared to the selenium plate of the priorart which'is generally not sensitive beyond 600 millimicrons, while atthe same time the plates of the invention have a greater quantumefiiciency, or in other words a generally increased sensitivity toradiation of the same wavelength.

Referring to FIG. 4, the relative spectral response of well-cycledplates is graphically illustrated when exposed to illumination having acolor temperature of 2800 K. By relative spectral response is meant therelative spectral photon intensity of the 2800 K. illumination times thequantum (photon) efficiency of the plate and which is plotted as theordinate while wavelength of the illumination is represented by theabscissa. Curve D represents the selenium test plate of the prior artwhile Curve E represents the average of the plates of the examples inaccordance with the invention. The overall plate speed is proportionalto the area under its curve from which it can be seen that substantialplate speed is gained with plates of the invention, particularly in therange of wavelengths between approximately 500 and 700 millimicrons.

Tests for electrical behavior were performed by exposing the chargedplates that had been well cycled to a 200 watt incandescent bulb througha range of aperture openings between f/ 4.5 and f/ 22. Initialsensitizing charges of negative polarity were applied to the plate ofthe examples whereas positive polarity charges were applied to seleniumplates of the prior art. After exposure the residual potential on eachplate was measured. The result of these tests are graphicallyillustrated by the curves of FIG. 5 in which residual plate potential isplotted as the ordinate and the log exposure of the various openings asthe abscissa. Curve F represents a plate in accordance with theinvention which has been sensitized with a negative polarity chargingpotential while curve G represents a commercial selenium plate havingbeen sensitized with positive polarity charging potential.

At the f/ll aperture opening in accordance with the tests of FIG. 5,potential on a plate of the invention was measured before exposureapproximately 5 seconds after charging and then measured after exposurewith and without regeneration, and the difference in potentials measuredbeing defined as the electrical contrast. It was found thatpregeneration in the instance of a first used plate or after a prolongedrest, or regeneration with a well-cycled plate of several hundredcycles, substantially increased the electrical contrast. Table Itabulates average characteristics of the plates without regenerationwhile Table II tabulates average characteristics with regeneration. Vrepresents the first measured charge while V is the contrast potentialdilference.

TABLE I After initial completion of the testing, the testing wasrepeated using duplicated batches of plates constructed similarly inaccordance with the examples. These plates were found to be similarlyresponsive, thus establishing reproducibility with reliability ofcharacteristics. Good results were found with selenium-tellurium layersof about .01 micron thickness to about 1.0 micron thickness. Goodresults were obtained with seleniumtellurium alloys in which the percenttellurium by weight varied between about 5% and about 40%. Optimumresults were obtained with approximately 20-28% tellurium by weight with25% being preferred. It has been found that the spectral responseextends further towards the red as the percentage of tellurium isincreased but that dark decay and residual potential increase withtellurium concentrations of over 25 thus reducing the availableelectrical contrast. Thickness of the selenium storage layer is usuallyon the order of between 10 and 300 microns.

A protective overcoating was found to preserve the plate characteristicsunder extended recycling although initially offered no perceptibleadvantage nor did it in any obvious way enhance the plate propertiesexcept to effect preservation. Protective layers other than thatdisclosed can be used in the alternative as for example an inorganiclayer such as disclosed in Owens US. Patent 2,886,434.

Pregeneration and regeneration were found to improve the electricalcontrast characteristics of the plates of the invention although thesesame plates when operated without regeneration and regeneration producedsufiicient electrical contrast for image reproduction.

In addition, some multiplication (i.e., quantum etficiency greater thanhas been observed in selenium plates according to the invention whensensitized with negative charging. This multiplication elfect comesabout by placing a transparent conductive electrode extremely close tothe surface of the plate rather than in the more conventional charging,exposing, and developing pro- 'cedure described herein. It is believedthat special pronegative charging yet it has not heretofore beenpossible to employ negative charging because of the relativelyunfavorable properties of the pure selenium storage layer.

The special selenium processing is required to make the plates adaptableto negative charging. Very brief processing of the type described has abeneficial eifect upon the properties of conventional selenium plateswhen used with positive charging, but it is believed that the moreextensive processing described herein is desirable to impart optimumselenium properties for negative charging.

What is claimed is:

1. An imaging method comprising:

(a) providing a xerographic plate having an electrical conductivesupport, a layer of long electron range vitreous selenium overlayingsaid support, and a layer overlying said vitreous selenium layercomprising a selinum-tellurium alloy containing between about to 40percent tellurium by weight;

.Selenium alloy plates may be particularly adapted to (b)electrostatically charging said plate to a negative potential;

(0) exposing said plate to a pattern of light and shadow to form alatent electrostatic image; (d) developing the plate withelectroscopically attractable material to form an image; (e)transferring the image to a support material; (f) charging said plate toa positive potential, whereby said plate is regenerated for further use.2. The method of claim 1 wherein during the positive charging step theplate is exposed to uniform illumination- References Cited UNITED STATESPATENTS 2,753,278 7/1956 Bixby et al. 117-200 2,803,541 8/1957 Paris96-1 2,858,239 10/1958 Nitsche 117-200 2,862,817 12/1958 Meyer et al.96-1 2,937,943 5/1960 Walkup 96-l 2,962,376 11/1960 Schaffert 96-13,041,166 6/1962 Bardeen 96--1 3,041,167 6/ 1962 Blakney et al 96-]3,057,719 10/1962 Byrne et al 961 3,077,386 2/1963 Blakney et al 96--1NORMAN G. TORCHIN, Primary Examiner. A. LIBERMAN, C. E. VANHORN,Assistant Examiners.

1. AN IMAGING METHOD COMPRISING: (A) PROVIDING A XEROGRAPHIC PLATEHAVING AN ELECTRICL CONDUCTIVE SUPPORT, A LAYER OF LONG ELECTRON RANGEVITREOUS SELENIUM OVERLAYING SAID SUPPORT, AND A LAYER OVERLYING SAIDVITREOUS SELENIUM LAYER COMPRISING A SELINUM-TELLURIUM ALLOY CONTAININGBETWEEN ABOUT 5 TO 40 PERCENT TELLURIUM BY WEIGHT; (B) ELECTROSTATICALLYCHARGING SAID PLAT TO NEGATIVE POTENTIAL; (C) EXPOSING SAID PLATE TO APATTERN OF LIGHT AND SHADOW TO FORM A LATENT ELECTROSTATIC IMAGE; (D)DEVELOPING THE PLATE WITH ELECTROSCOPICALLY ATTRACTABLE MATERIAL TO FORMAN IMAGE: (E) TRANSFERRING THE IMAGE TO A SUPPORT MATERIAL: (F) CHARGINGSAID PLATE TO A POSITIVE POTENTIAL, WHEREBY SAID PLATE IS REGENERATEDFOR FURTHER USE.